High heat capacity heat transfer fluids are used in several industries to provide cooling for shell and tube heat exchanger arrangements. Various types of high heat capacity fluids include alkali liquid metals such as sodium, lithium, and potassium and include molten salts such as nitrates and carbonates. These heat transfer fluids combine high heat capacity with high thermal conductivity. British patent 2170898 generally discloses the use of sodium as a heat transfer medium in high temperature reactions including heat recovery from furnace installations, high pressure nuclear reactors, coal gasification, coal conversion, and water disassociation. U.S. Pat. No. 4,549,032 discloses the use of molten salt as an indirect heat transfer medium with a dehydration of styrene. German patent DE 2028297 discloses the use of an alkaline metal as a heat transfer medium in a process for producing alkenes and aromatics by cracking aliphatic hydrocarbons. The liquid metals are specifically used due to their high heat transfer capacity that permits utilization of small heating surfaces.
Liquid metals are receiving more attention in many industries, like the petrochemical and chemical industries, where contact of reaction fluids with a catalyst in a reactor under suitable temperature and pressure conditions effects a reaction between the components of one or more reactants in the fluids. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone. Indirect heat exchange to maintain an isothermal or other temperature profile within the reaction zone can be particularly effective. It is known to accomplish indirect heat exchange for processes with a variety of heat exchanger configurations including shell and tube heat exchange designs or thin plates that define reaction and heat exchange channels. In such arrangements the tubes typically contain catalyst while the channels contain a heat exchange fluid or in a plate arrangement the channels alternately retain catalyst and reactants in one set of channels and a heat transfer fluid in adjacent channels. A specific arrangement for heat transfer and reactant channels that offers more complete temperature control can again be found in U.S. Pat. No. 5,525,311; the contents of which are hereby incorporated by reference. Other useful plate arrangements for indirect heat transfer are disclosed in U.S. Pat. No. 5,130,106 and U.S. Pat. No. 5,405,586.
These different types of heat exchange arrangement can benefit from the use of liquid metal heat transfer fluids that are circulated in a closed heat transfer loop. During the initial fabrication and at subsequent times when the heat exchange loop is opened to the atmosphere for maintenance an oxide layer can form on the metallic heat exchange surfaces. The liquid metal removes this oxide layer from the heat exchange surfaces thereby introducing oxides into circulating liquid metal. These oxides and other deposits that accumulate during startup and maintenance, sometimes referred to as the dirt burden, need to be removed to prevent precipitation of the impurities from the circulating liquid metal stream. Should the metal oxide or other impurity concentration exceed solubility limits, the precipitation of solids can interfere with the operation of the process or cause damage to equipment.
The need to efficiently and effectively deal with this dirt burden increases as more efficient and expanded heat transfer surfaces are employed. The trend in heat exchange arrangements for hydrocarbon conversion processes is to raise heat exchange efficiency by increasing the surface area for the indirect heat through the use of a series of thin stacked plates. This approach maximizes surface area, but as the area of the heat exchange surface increases so do the sites for initial oxidation which adds to the initial dirt burden and complicates its removal.
Those skilled in the art of using liquid metals as indirect heat exchange materials have addressed the problem of eliminating impurities, in particular, oxides and hydrides from the liquid metal streams. Two methods are routinely used, alone or in combination, to remove such impurities. One method removes impurities by precipitation and filtration or collection of the impurities in a cold trap. The typical apparatus associated with a cold trap comprises an economizer exchanger that transfers heat between hot, unpurified metal and the cold purified metal, a cooler for the liquid metal, and some form of retainer for a filtering element. U.S. Pat. No. 4,928,497; U.S. Pat. No. 3,873,447 and U.S. Pat. No. 4,892,653 disclose different designs for cold traps and methods for their operation. U.S. Pat. No. 3,941,586 teaches the purification of cold trap by heating sodium hydride to a molten state and removing or venting hydrogen gas from the cold trap and also describes the presence of a drain tank. Most systems for the circulation of a liquid sodium heat transfer fluid include a dump or drain tank that can hold the circulating inventory of liquid sodium when the system is shut down.
The other method of removing impurities chemically reacts the impurities with a chemisorbent or getter material that acts a scavenger to chemically bind an impurity or a substituent of the impurity to a fixed substrate that retains the reactive moiety of the getter. U.S. Pat. No. 4,830,816 discloses a getter type trap for removal of oxides from liquid metals. Suitable oxide getters use well known oxygen scavenging material such as zirconium, calcium, or titanium. Ordinarily the getter material is also retained in a dedicated trap that physically binds a mesh or gauze material made up of the getter material.
Managing the large dirt burden routinely calls for the use of such traps in a sacrificial manner somewhere in the flow path of the circulating heat transfer fluid. The trap will reach its sorption capacity shortly after the startup of the system and becomes useless thereafter. Once the impurity is chemically bound there is no need to continue passing the entire inventory through the getter material. Such passage raises pressure drop and needlessly expands the piping in the circulation loop. It is therefore advantageous to remove the trap from the system after its initial use and helpful to replace the trap to retain a capacity for oxide removal. Replacement of cold traps and getter material is costly and inconvenient.
Therefore, it is particularly desirable to have a process that can simply and effectively handle a high dirt burden. Accordingly it is an object of this invention to improve methods for removing the dirt burden of oxides and other materials from a circulating liquid metal heat exchange fluid. It is a further object of this invention to eliminate the need for removing and replacing in line oxide traps from a system for circulating a liquid metal heat transfer fluid.